Semiconductor devices with high carrier mobility are required for high speed communications in frequency ranges from about 1 giga-hertz (GHz) to about 100 GHz. Expensive compound semiconductor materials systems such as InP, GaAs, and GaN are often used for such devices. Typically, these material systems are not compatible with high volume, Si-based CMOS manufacturing processes and cannot benefit from the economies of scale that have been achieved therein. Therefore, the manufacturing cost and systems cost for communication networks using such devices remains high and limits the adoption and deployment of such networks.
Semiconductor devices with semiconductor layers based on strained silicon and/or Si—Ge alloys exhibit properties that are suitable for use in communication networks at frequencies between about 1 GHz to about 75 GHz. Furthermore, these materials are generally compatible with high volume, Si-based CMOS manufacturing processes. Therefore, these material systems are attractive alternatives to the compound semiconductor materials systems listed above.
The strained silicon and/or Si—Ge alloy materials must meet a number of requirements. Ideally, the materials should be deposited epitaxially (i.e. as a single crystal) with a low density of defects such as grain boundaries, dislocations, point defects, etc. Defects serve as scattering sites and lower the mobility of the carriers within the material. High device speeds can be obtained by increasing the concentration of Ge in a Si—Ge alloy. However, at high Ge concentrations and higher temperatures typically used for the thermally activated growth of Si—Ge alloys, the films relax at a critical thickness resulting in a highly defective film. Additionally, Ge tends to segregate or cluster within the film, forming a non-uniform and highly defective film. Other disadvantages of high temperature growth include dopant redistribution, dopant surface segregation, autodoping, and incompatibility with low temperature substrates such as thin film solar cells, thin film transistors, polymers, etc. These disadvantages reduce the mobility of the carriers within the device channel and lower the device speed. Simply lowering the growth temperature to about 550 C results in issues such as a slow deposition rate, and high defect density.
Attempts have been made to address the issues discussed above by depositing epitaxial Si, Ge, and/or Si—Ge alloys using plasma enhanced chemical vapor deposition (PECVD) techniques. Typically, PECVD techniques can be applied at lower temperatures and use plasma energy to drive the chemical reactions. Typically, the various precursor and reactant species are exposed to the plasma region of the reactor. This is generally true for both direct plasma and remote plasma configurations. The plasma energy interacts with the various gaseous species to form ions, electrons, radicals, and energetic neutral species. These various species can interact in the gas phase through unwanted reactions and can form particulate matter that is deposited on the substrate rather than forming the desired film through well controlled surface reactions. Therefore, PECVD has many challenges when applied to the epitaxial growth of Si, Ge, and/or Si—Ge alloys.
Therefore, there is a need to develop methods and apparatus that allow the low temperature growth of Si, Ge, and/or Si—Ge alloys with high Ge concentrations and low defect densities.